Process for installing a helical added thread in an internally threaded orifice for repairing the internal threads of the orifice

Abstract

To repair the internal thread (2b) of a large diameter orifice, an added thread is screwed into the orifice in a form wound on a threaded part of the surface of a drum screwed into an internally threaded centering ring engaged in the entry part (2a) of the orifice (2). The thread is laid against the internal thread (2b) and fastened in place by installing a pin passing through the thread and engaged in the wall of the orifice (2). The process is used for repairing the internal threads of orifices (2) machined in the flange of a pressurized-water nuclear reactor vessel, allowing the vessel cover to be bolted down sealingly.

We claim: 
     
       1. Process for installing a helical added thread in a large diameter orifice for repairing an internal thread of said orifice, said process comprising the steps of (a) providing a cylindrical drum having an at least partly threaded external surface;   (b) engaging and winding said added thread on a threaded part of said external surface of said drum;   (c) providing a centering ring having a threaded internal surface and an engagement end part;   (d) engaging and maintaining said engagement end part of said centering ring in an entry part of said orifice, so that said threaded internal surface of said centering ring constitutes an extension of said internal thread of said orifice;   (e) screwing said drum on which said added thread is wound onto said threaded internal surface of said centering ring and then into said internal thread of said orifice;   (f) laying said added thread against said internal thread of said orifice; and   (g) fastening said added thread in place by passing a pin through the thread and engaging said pin in an internal wall of said orifice.   
     
     
       2. Process according to claim 1, wherein said added thread consists of a material having a hardness substantially higher than the hardness of the material forming said wall of said orifice and of the material of a fastening element to be screwed into said orifice. 
     
     
       3. Process according to claim 1 or 2, wherein said added thread consists of a winding with continuous turns, said winding having the length which corresponds substantially to a length of the thread of said internal thread which is being repaired. 
     
     
       4. Process according to claim 1 or 2, wherein said added thread has an end flight bent towards the inside of said added thread in a substantially radial arrangement. 
     
     
       5. Process according to claim 1 or 2, wherein said thread has a cross-section in the form of a parallelogram with rounded angles.